While the neurons and glial cells of the retina are known to arise from multipotent precursor cells during embryonic development, the environmental signals that regulate their proliferation, differentiation and maturation are not fully understood. The neurotrophins are a family of proteins that have been shown to influence survival, proliferation, differentiation and maturation in the developing nervous system. The biological activities of these neurotrophins are mediated by two classes of cell surface receptors: the trk family of receptor tyrosine kinases and the neurotrophin receptor p75. In immunohistochemical studies, the applicant has shown that during retinal development in the chick, expression of the neurotrophin NT-3 overlaps spatially and temporally with that of its cognate receptor, trkC. This coordinated expression of receptor and ligand is consistent with the hypothesis that NT-3 functions locally with the developing retina. Since the trkC gene undergoes alternative splicing, resulting in the expression of several isoforms which differ with respect to their signaling capabilities and represent an important mechanism for regulating trkC function, the applicant proposes to describe the expression of truncated and kinase insert isoforms of trkC by immunohistochemistry, in situ hybridization and RT-PCR analysis. In addition, to test the hypothesis that NT-3 influences the development of the retina, gene transfer by replication-defective retroviruses will be used to alter the expression of NT-3 and trkC receptor isoforms in the developing chick retina. Specifically, vectors encoding NT-3 and the full-length isoform of trkC will be used to overexpress NT-3 and trkC, respectively, thus upregulating NT-3:trkC signaling. Conversely, vectors encoding a truncated isoform of trkC will cause downregulation of NT-3:trkC signaling. The effects of misexpression of NT-3 and trkC on cell proliferation and survival, on the differentiation of retinal cell types and on the formation of synapses in the developing chick retina will be determined. The long-term goal of this work is to define the functions of NT-3 during retinal development and to determine whether abnormalities in NT-3 function result in retinal defects. An understanding of the functions of neurotrophins in the retina could also be relevant to therapies for conditions involving ganglion cell loss such as glaucoma. The use of neurotrophins in conjunction with gene therapy or transplantation may also have potential for reversing retinal degenerative changes and lead to the restoration of vision.